Tunable low noise amplifier and current-reused mixer for a low power RF application

ABSTRACT

A radio frequency front-end receiver includes a single stage low noise amplifier connected with a resistor array and a capacitor array, and a Gilbert-type mixer connected with a PMOS transconductance stage, an inductor and a serially connected current source. The resistor array enables the adjustment of the power gain of the low noise amplifier. The capacitor array tunes the low noise amplifier so that the maximum power gain is at the desired operating frequency. The PMOS transconductance stage reduces the power consumption of the mixer. The inductor increases the impedance and the current source improves the common-mode rejection of the mixer.

FIELD OF THE INVENTION

The present invention generally relates to a radio frequency (RF) frontend receiver in wireless communication, and more specifically to the lownoise amplifier and mixer in the RF front end receiver for improving itsperformance.

BACKGROUND OF THE INVENTION

FIG. 1 shows a block diagram of the receiver end of a conventionalwireless RF system. The RF system comprises an antenna 10 for receivinga weak RF signal which is first amplified by a low noise amplifier 120in the receiver 12. The amplified RF signal is then mixed in a mixer 124with the output of a local oscillator 14 to form a mixed signal thatincludes both intermediate frequency and microwave signals.

A conventional low noise amplifier normally includes two stages in orderto achieve the goal of adjusting the power gain of the low noiseamplifier. FIG. 2A shows the first stage of a conventional low noiseamplifier which comprises a first inductor 20, a second inductor 21, afirst MOS transistor 22 and a second MOS transistor 23. The secondinductor 21 serves as a serial feedback to match and reduce the noise ofthe low noise amplifier. The second stage that is not shown in FIG. 2Ais used to adjust the power gain of the amplifier.

Because of the two stage circuit design, the conventional low noiseamplifier has the drawbacks of high power consumption and complicateddesign. Furthermore, the second inductor 21 in the conventionalamplifier is often designed into the architecture of a single chip.However, this inductance value can not be too small. In order toincrease the power gain of the low noise power amplifier, the qualityfactor Q values of the first and second inductors 20, 21 have to besufficiently large. Therefore, external inductors with high qualityfactor Q values are required. This makes the adjustment of theefficiency of the low noise power amplifier very difficult.

FIG. 2B shows the circuit of a conventional Gilbert-type mixer 124. TheRF signal is modulated by an oscillation signal in a transconductancestage formed by a third MOS transistor 28 and a fourth MOS transistor 29to accomplish the effect of wave mixing. The oscillation signal of alocal oscillator (LO) enters the mixer 124 through a switching circuitcomprising a fifth MOS transistor 24, a sixth MOS transistor 25, aseventh MOS transistor 26 and an eighth MOS transistor 27.

A drawback of the Gilbert-type mixer 124 is that it can not improve thesignal to noise ratio and reduce the power consumption at the same time.The current in the switching circuit is proportional to the current inthe transconductance stage. The input noise is proportional to thecurrent in the switching circuit but inversely proportional to thecurrent in the transconductance stage. Because of the relationship amongthe input noise, the current in the switching circuit and the current inthe transconductance stage, it is necessary to increase the current inthe transductance stage as well as decrease the current in the switchingcircuit in order to reduce the noise. However, the two requirements arecontradictory. The only choice is to increase the power consumptionwhich also increases the noise in the switching circuit. Therefore, atrade-off has to be made between low noise and high power consumption ina conventional mixer. It is very difficult to design a mixer with bothlow noise and low power consumption.

SUMMARY OF THE INVENTION

This invention has been made to overcome the above mentioned drawbacksof the conventional low noise amplifier and mixer used in the wirelesscommunication. The primary object of this invention is to provide aradio frequency front-end receiver that has the advantage of both lownoise and low power consumption.

Accordingly, the radio frequency front-end receiver of the presentinvention comprises an improved single stage low noise amplifier and animproved Gilbert-type mixer. The single stage low noise amplifier has aresistor array connected thereon for adjusting the power gain of theamplifier. It also has a capacitor array connected thereon for tuningthe amplifier so that the maximum power gain of the low noise amplifieris at the desired operating frequency.

The improved Gilbert-type mixer includes a PMOS transconductance stagethat comprises two PMOS transistors for reducing the power consumptionwithin given power gain and circuit linearity. An inductor is connectedto the Gilbert-type mixer in order to provide high impedance under theoperating frequency. The Gilbert-type mixer is also connected in serialwith a current source circuit that has a pair of MOS transistors toincrease the input impedance as well as improve the common-moderejection ratio of the mixer.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become better understood from a careful readingof a detailed description provided herein below with appropriatereference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the radio frequency front-end receiver of a conventionalwireless communication system.

FIG. 2A shows the circuit diagram of a conventional low noise amplifier.

FIG. 2B shows the circuit diagram of a conventional Gilbert-type mixer.

FIG. 3A shows the circuit diagram of the low noise amplifier accordingto the present invention.

FIG. 3B shows the circuit diagram of the mixer according to the presentinvention.

FIGS. 4A and 4B illustrate the test performance of the radio frequencyfront-end receiver of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 3A shows the circuit of the low noise amplifier according to thepresent invention. As can be seen from the figure, a resistor array 30and a capacitor array 31 are connected to the first stage low noiseamplifier 32 illustrated in FIG. 2A of a conventional two stageamplifier. The resistor array 30 comprises a plurality of resistordevices connected in parallel. Each resistor device includes a resistor300 connected in serial with a switch 301. One end of the resistor array30 is connected to the positive voltage VDD and the other end isconnected to a first terminal of a first inductor 20. The equivalentresistance of the resistor array 30 can be varied by switching on or offsome of the switches 301 in order to adjust the desired power gain ofthe low noise amplifier of the present invention.

The capacitor array 31 comprises a plurality of capacitor devicesconnected in parallel. Each capacitor device includes a capacitor 310connected in serial with a switch 311. One end of the capacitor array 31is connected to the ground and the other end is connected to a secondterminal of the first inductor 20. Similarly, the equivalent capacitanceof the capacitor array 31 can be varied by switching on or off some ofthe switches 311 to tune the low noise amplifier so that the maximumpower gain is at the operating frequency.

The low noise amplifier according to the present invention has severaladvantages. First of all, the design is simple and the power consumptionis low because the circuit has only one stage. Secondly, only the secondinductor 21 has to be a device external to the single chip of the lownoise power amplifier. Therefore, the noise interference to the signalin the single chip is greatly reduced. Thirdly, when the power gain isadjusted, the input impedance is not affected. Furthermore, the layoutrouting of this design is parasitic insensitive.

The resistor array and the capacitor array in the present invention canbe controlled either digitally or in analogue. For example, instead ofconnecting a switch to a resistor in a resistor device of the resistorarray 30, the resistor device can also be a voltage controlled variableresistor. Each capacitor device in the capacitor array 31 can also bereplaced by a voltage controlled variable capacitor. In the low noiseamplifier, the connection of the first and second inductors 20, 21 tofirst and second MOS transistors 22, 23 is the same as in a conventionallow noise amplifier.

FIG. 3B shows the circuit of the mixer according to the presentinvention. As can be seen from the figure, a PMOS transconductance stage33, a third inductor 34 and a serially connected current source circuit35 are connected to a conventional Gilbert-type mixer. The PMOStransconductance stage 33 comprises a first PMOS transistor 330 and asecond PMOS transistor 331. The source of the first PMOS transistor 330is connected to the source of the second PMOS transistor 331 as well asone end of the third inductor 34. The gate of the first PMOS transistor330 is connected to the gate of a third MOS transistor 28. The drain ofthe first PMOS transistor 330 is connected to the drain of the third MOStransistor 28.

The gate of the second PMOS transistor 331 is connected to the gate of afourth MOS transistor 29. The drain of the second PMOS transistor 331 isconnected to the drain of the fourth MOS transistor 29. The function ofthe PMOS transconductance stage 33 is to reduce the power consumption ofthe mixer within given gain and linearity. The function of the thirdinductor 34 is to provide high impedance at the operating frequency. Inaddition, the third inductor 34 also has the advantage of common-moderejection and low operating voltage because it has no direct currentvoltage drop.

The current source circuit 35 connected in serial includes a pair of MOStransistor. One end of the current source circuit 35 is connected to thesource of the third MOS transistor 28 and the source of the fourth MOStransistor 29. The other end of the current source circuit 35 isconnected to ground. The two MOS transistors in the current sourcecircuit 35 not only provide high output impedance but also increase thecommon-mode rejection ratio of the mixer. Therefore, the mixer has goodability to reject common-mode noise.

The receiver of this invention has a single input and differentialoutputs. It eliminates the need to have a single input/different outputconverter. Because the invention employs current-reused circuittopology, the low noise amplifier of this invention requires only halfof the current used in a conventional mixer under the same power gainand linearity with at least 3 dB improvement of noise. Therefore, theinvention consumes only about half of the power consumed by aconventional mixer.

The current in the transconductance stage can be increased and thecurrent in the switching circuit of a conventional Gilbert-type mixercan be decreased to reduce the noise. Because the power gain, noisereduction and circuit linearity can be optimized separately, the presentinvention achieves the advantages of both low noise and low powerconsumption without compromise.

FIGS. 4A and 4B illustrate the performance of the radio frequencyfront-end receiver according to the present invention. FIG. 4A shows theconversion gain of the receiver. Within the 2.4˜2.5 GHz radio frequencyrange, the present invention has a gain of approximately 35 dB. FIG. 4Bshows the noise figure of the receiver. Within the same radio frequencyrange, the noise figure of the receiver is about 1.9 dB.

Although the present invention has been described with reference to thepreferred embodiments, it will be understood that the invention is notlimited to the details described thereof. Various substitutions andmodifications have been suggested in the foregoing description, andothers will occur to those of ordinary skill in the art. Therefore, allsuch substitutions and modifications are intended to be embraced withinthe scope of the invention as defined in the appended claims.

1. A radio frequency front-end receiver for wireless communication,comprising: a single stage low noise amplifier having a resistor arrayfor adjusting power gain of said receiver, and a capacitor array fortuning said receiver to achieve optimal gain at an operating frequency;and a mixer having a PMOS transconductance stage, a high impedanceinductor and a serial current source connected to a Gilbert-type mixer,said PMOS transconductance stage including first and second PMOStransistors, and said serial current source including a pair of MOStransistors; wherein said first and second PMOS transistors reduce powerconsumption of said mixer under given gain and linearity constraints,said high impedance inductor provides high impedance at said operatingfrequency, and said pair of MOS transistors provides high outputimpedance and a high common-mode rejection ratio for said mixer.
 2. Theradio frequency front-end receiver for wireless communication as claimedin claim 1, wherein said resistor array comprises a plurality ofresistor devices connected in parallel, each of said resistor deviceshaving a resistor connected in serial with a switch.
 3. The radiofrequency front-end receiver for wireless communication as claimed inclaim 1, wherein said resistor array comprises a plurality of resistordevices connected in parallel, each of said resistor devices having avoltage controlled variable resistor.
 4. The radio frequency front-endreceiver for wireless communication as claimed in claim 1, wherein saidcapacitor array comprises a plurality of capacitor devices connected inparallel, each of said capacitor devices having a capacitor connected inserial with a switch.
 5. The radio frequency front-end receiver forwireless communication as claimed in claim 1, wherein said capacitorarray comprises a plurality of capacitor devices connected in parallel,each of said capacitor devices having a voltage controlled variablecapacitor.
 6. The radio frequency front-end receiver for wirelesscommunication as claimed in claim 1, wherein said single stage low noiseamplifier comprises a first inductor, a first MOS transistor, a secondMOS transistor and a second inductor connected in serial.
 7. The radiofrequency front-end receiver for wireless communication as claimed inclaim 6, wherein a first end of said first inductor is connected to saidfirst MOS transistor, and said resistor array has a first end connectedto a second end of said first inductor and a second end connected to apositive voltage supply.
 8. The radio frequency front-end receiver forwireless communication as claimed in claim 7, wherein said capacitorarray has a first end connected to said first end of said first inductorand a second end connected to ground.
 9. The radio frequency front-endreceiver for wireless communication as claimed in claim 6, wherein saidfirst PMOS transistor has a source connected to a source of said secondPMOS transistor and a first end of said high impedance inductor, a gateconnected to a gate of a third MOS transistor, and a drain connected toa drain of said third MOS transistor, and said second PMOS transistorhas a gate connected to a gate of a fourth MOS transistor, and a drainconnected to a drain of said fourth MOS transistor.
 10. The radiofrequency front-end receiver for wireless communication as claimed inclaim 9, wherein said serial current source has a first end connected toa source of said third MOS transistor and a source of said fourth MOStransistor, and a second end connected to ground.
 11. The radiofrequency front-end receiver for wireless communication as claimed inclaim 6, wherein said mixer further comprises fifth, sixth, seventh andeighth MOS transistors.